The present invention relates to components for optical communications systems, specifically interconnection and coupling devices utilizing multilayer optical interference filters for group delay and dispersion compensation.
Optical communications systems comprise an interconnected network of optical fibers for transmitting a plurality of the optical signal channels between nodes in the network. In order to increase the capacity of existing optical communications systems, or provide for flexible reconfiguration, multiple optical signal channels may propagate between nodes simultaneously using time division and wavelength division multiplexing (WDM). Wavelength division multiplexing refers to a plurality of signal channels characterized by a different wavelength of light, while time division multiplexing refers to a time sequence allocation of digital signals within a common optical signal channel. Although information may be transmitted in analog format in a WDM system, the digital format is commonly used in telecommunications because of the higher data transfer rates and compatibility with time division multiplexing schemes deployed in electronic communications systems.
As a WDM communication system utilizes optical signals of different wavelengths the optical fiber network must be configured such that the time sequential nature of information traveling on different wavelengths between common nodes is not temporally distorted. While such temporal distortion is influenced by design and environmental factors, it is frequently due to the wavelength dependence of the refractive index within the optical media forming the waveguiding optical fiber. The velocity of light is decreased on transmission through a dense media, such as optical glass fibers, in proportion to the refractive index ratio between free space transmission, 1, and the refractive index of the optical glass at the signal channel wavelength ng. As a refractive index of glasses vary with wavelength,λ, (i.e. ng=n (λ) optical signals will be distorted, that is distributed in arrival time at the terminal node in the communication system network in proportion to the distance between originating node and the terminal node. The change in refractive index of a material with wavelength is commonly referred to as chromatic dispersion. Thus, as the distance between nodes in the optical communication system increases, or the digital pulse width decreases in order to obtain greater signal transfer capacity, the inherent properties of optical glasses become a greater limitation on performance and reliability.
Chromatic dispersion of optical fiber is roughly constant over the 1550 nm communication window, and can be compensated by several techniques including dispersion compensating fiber, which has a radial gradient in refractive index to provide self correction, Fiber Bragg gratings, etc. However, certain wavelength filtering components such as multilayer interference filters (MLIF) can have significant dispersion characteristics due to a fundamental Kramers-Kronig type relationship between transmission spectrum and dispersion characteristics. This type of temporal distortion is also quantified as the group delay (GD), typically in units of pico-seconds (ps). As the GD characteristic varies substantially with wavelength over the narrow passband (that is the high transmission region corresponding to the allocation of signal channels at specific wavelength per ITU convention) of an MLIF, the derivative of GD with respect to wavelength is also denominated or characterized as the dispersion, typically in units of ps/nm (pico-seconds/nanometer). Such dispersion is difficult to compensate using conventional techniques such as dispersion compensating fiber.
Other sources of signal temporal distortion may arise for various active or passive components within the optical communication network, such as optical amplifiers, multiplexing filters, gain flattening filters, arrayed waveguides, Fiber Bragg gratings and the like, as well as temperature fluctuations. Accordingly, as an optical communication system is reconfigured for repair, maintenance or to meet changes in demand, the temporal distortion of signals may change in a manner that is not easily predictable. Numerous methods providing for the effects of GD, whether arising through the characteristics of the optical fiber or system components, have been developed. These methods include devices that either provide a fixed amount of compensation or an adjustable amount of compensation, or may be deployed at or between nodes in the optical communication system.
As new interconnections are required to insert such devices within the optical communication system it is desirable that the devices themselves, as well as the connections thereto, result in a minimum signal loss.
U.S. Pat. No. 5,557,468 in the name of Ip assigned to JDS Fitel Inc, of Nepean Canada issued Sep. 17, 1996 and shows a dual Gires-Tourneau (GT) etalon dispersion compensator. This '468 patent states that cascading two filters having the same reflectivity on the input/output mirrors has been suggested, but does not produce optimum results with respect to increasing the wavelength region over which the equalizer operates; The Ip patent illustrates that by cascading a first etalon with a second etalon having dissimilar reflectivity characteristics and being slightly offset in its center frequency response, it is possible to favorably extend the range of the output response. Although Ip's two etalons achieve their intended purpose of extending the operation wavelength range, it would be advantageous to have a device that provides a controllable constant amount of dispersion within a wavelength band of interest. That is, where tuning allowed different constant amounts of dispersion to be induced. Indeed, in it is also of interest to provide various types of dispersion compensation to a wavelength region of interest that corresponds to only a single optical signal channel. It is also of interest to provide an arbitrary dispersion compensation across a wavelength region as narrow as a single optical signal channel to permit matching of the dispersion of combined optical signal channels at different wavelengths; this is particularly true in instances where the commonly propagating optical signal channels will be subject to an additional compensation scheme.
Reflective MLIF's that provide dispersion or group delay compensation have been disclosed in U.S. Pat. Nos. 5,734,503; 6,301,049; 6,301,042; 6,222,673; 6,154,318 and 6,081,379, which are incorporated herein by reference. However, these solutions are inapplicable to a WDM system as they do not provide for single channel compensation and/or attenuate or reject the other optical signals, that is those not intended to be compensated. Further, these thin film design approach all achieve the desired group delay or dispersion control on reflection from dielectric stacks wherein the optical thickness of the quarterwave layers is perturbed to improve the group delay over the reflective stacks bandwidth. These filter designs are inapplicable not only when the modification of only a narrow bandwidth is desired, but also do not generally provide the level of GD or dispersion correction required in an optical communication system.
Accordingly, yet a further objective of the invention is to provide dispersion correction devices that are simple, compact and thermally stable.
It is also an objective of the present invention to compensate for the dispersion from WDM devices that comprise MLIF's, including multiplexers, demultiplexers, gain flattening filters and interleavers.
Accordingly, it is an object of the present invention to provide a MLIF that provides for chromatic dispersion compensation of an individual optical signal channel in the ITU grid.
It is also an object of this invention to provide a dispersion compensator that will provide a certain amount of dispersion over a predetermined wavelength band.
Yet another object is to provide, fixed, variable and non-linearly variable GD and dispersion compensation over a wavelength range corresponding to a single optical signal channel, as well as multiple optical signal channels.
It is a further object of the invention to provide devices and methods that permit tunable levels of GD and dispersion compensation or correction with a reduced of signal loss.
It is another object of the invention to provide a dispersion compensator that will provide tunable dispersion compensation over a range of wavelengths.